Are there any rules how to build the diminutive?

Question

We build a diminutive by adding the suffix -lein or -chen. For some nouns one is preferred over the other whereas with other nouns both may be used.

Examples:

der Tisch | das Tischlein | das Tischchen
das Brot  |       -         das Brötchen
der Bach  | das Bächlein  |     -

In Hochdeutsch there is a tendency to prefer -chen whereas regionally the suffix corresponding the -lein-form (e.g. -li Swiss, -le Swabian) is used.

Apart from that, are there any rules that define which suffix to use?

Answer 1

In my experience, for Hochdeutsch, "-lein" nowadays is often the exception that you would fall back to in case the more common "-chen" doesn't work, cf. "Bächlein" (because "Bächchen" would be impossible to pronounce even for native speakers).

An a/o/u vowel often changes to their corresponding umlaut:

Bach -> Bächlein

Brot -> Brötchen

Punkt -> Pünktchen

Additionally, if the last letter is a vowel, it is often omitted when forming a diminutive:

Schraube -> Schräubchen

Katze -> Kätzchen

There's also a separate diminutive "-i/-y" form mainly used with person names:

Hans -> Hansi

Fred -> Fredi

Thomas -> Tommy

Some examples of both Hochdeutsch and dialects can be found on Wikipedia

Answer 2

As you mentioned, in Switzerland we use only "li". If the last vovel is "a" "o" or "u" it gets to the corresponding umlaut "ä" "ö" or "ü"

• Hund: Hündli
• Brot: Brötli
• Elefant: Elefäntli

But there are also exceptions like

• Sou : Söili
• Bogä: Bögeli/Bögli